Toto Guinevere tank filling past overflow tube

Greetings. I had my bathroom gutted and remodeled almost two years ago. While I purchased the new tub and sink from the local plumber who did the installation, I researched here and online elsewhere about the best low-flow toilet I could get. My old toilet was a 3.5 gal. I chose the Toto Guinevere which I ordered online and had shipped to me. I was quite satisfied with this toilet and until now it has given me no issues.

Some people have complained about the tinkling and dripping sounds it makes for several minutes after each use. This has never bothered me, and in fact, it is because of this dripping noise that I discovered just today that a problem is brewing.

Earlier, after not having used the toilet in awhile, it suddenly began making the dripping noise. It didn't stop. The water level in the bowl remained full. I took the lid off of the tank and saw that water was slowly streaming from the black refill tube into the overflow pipe. It was like a very fast trickle but not quite a full stream. I flushed the toilet. The tank refilled and the water coming from the refill tube had completely stopped.

About 15 minutes later, I heard the dripping noises again. I again removed the tank lid. This time the refill line was not dripping at all, but the water level in the tank had slowly risen and was going slowly over the overflow pipe. Hence the same result but from a different method.

I flushed the toilet again. The tank refilled and stopped where it normally does, about an inch or so below the top of the overflow pipe. But about 15 minutes later, the water level in the tank had risen again. For now I have the water line to the toilet shut off.

Anyone have any idea as to what it is that is going bad inside my tank?

Toto uses two suppliers for their flush valves. If the one you have is made by Korky (it has a removeable top, the other one has a plastic screw on the top), you can pick up a seal at any plumbing supply, most hardware stores, and Lowes for about $2. It takes about 30-seconds to replace once you've done it the first time. There may be some crud caught underneath the sealing surface, but once you have it out, it's often easier to just replace it rather than try to clean it. the instructions are on the package.

If you have excessive water pressure, things like this will leak when they start to age. You might also want to pick up a pressure gauge, screw it onto some place like a washing machine supply, the drain on the WH, a utility sink, or maybe an outside faucet (if they aren't shut off for the winter). Leave it for 24-hours and make sure you get one with a second, tattle-tale hand to record peak pressure. If the static or peak pressure is greater than 80psi, you need a PRV and an expansion tank. That will help the life of all valves in the house.

My green fill valve has a small white plastic screw. It says on top "Cyclone Flushing System By TOTO" but it has no brand name that I can see. I noticed that on the inside of the tank lid it has a large sticker with a diagram and a list of all part numbers. Will replacing the fill valve solve this issue? I do recall reading in the installation instructions a couple of years ago that there would be one of two possible types of fill valves, and that one of the two was adjustable while the other was not, if I am recalling things correctly from my memory.